A need for real-time solder paste process control is critical due to the dynamic nature of solder paste. Both the rheology and the solderability of solder paste can change drastically during manufacturing. These dynamic changes are dependent on the manufacturing environment and on the characteristics of the specific paste being used. An environment with high .humidity can cause an increase in slump and, potentially, an increase in the probability of solder balls due to absorbed moisture. In addition, solder paste, over time, can either increase or decrease in viscosity; this classifies the paste as being thixotropic or rheopectic, respectively. The dynamic change in rheology can cause significant problems in the printability and slump of the solder paste. Lastly, any changes in the flux material can effect the solderability of the solder powder and can also have an impact of the rheologic nature of the paste due to the excessive build-up of reaction products between the flux activators and the metal oxides (such as S.sub.n O and/or S.sub.n O.sub.2).
Current methods of measuring the rheologic characteristics of solder paste entail the use of a viscometer. A viscometer is capable of measuring the viscosity of a solder paste material at a different shearing rates. Thus, the viscosity of a solder paste can be tracked at a reference shear rate and the thixotropic character of the solder paste can be tracked by calculating the change in viscosity over a change in shearing rate. Currently, there are two viscometers commonly used in the industry: a Malcolm Viscometer and a Brookfield Viscometer. Both of these systems allow a manufacturer to quantify both viscosity and thixotropic behavior.
The Brookfield viscometer uses a T-type spindle that rotates at a given rate in rotation and z-height while the Malcolm uses a screw-type spindle that causes the solder paste to pump up through the spindle to make a torque/viscosity measurement. The advantage of the Brookfield is in its acceptance by the industry and its maturity in quantifying viscosity. Due to the lack of controlled shearing with the T-type spindle, the Brookfield has limitations in measuring thixotropic behavior. The Malcolm is a relatively new viscometer design that was centered around the needs of solder paste rheologic measurements. The Malcolm is well designed to handle both viscosity and thixotropic measurements but does not have the same acceptance as the Brookfield in the electronics manufacturing industry. In both cases, neither system is capable of measuring the rheologic properties of solder paste once placed in a manufacturing environment. These systems are principally designed to make bulk rheologic measurements as an incoming inspection tool and typically require a significant amount of paste for an accurate measurement.
There are no other known electrical systems that measure and control solder paste materials currently available to the industry. Related U.S. Patents include U.S. Pat. No. 5,103,181 issued Apr. 7, 1992 to Gaisford et al. which is a composition and monitoring process that uses impedance measurements. In U.S. Pat. No. 4,939,469 a method for the evaluation of printed circuit boards is disclosed that uses the impedance spectra of the board to evaluate a number of characteristics such as moisture content. And U.S. Pat. Nos. 3,482,161, 3,440,529, and 3,448,380 all use spectroscopic analysis for sample analysis.